What is the graph called when the time waveform is converted using FFT?

The correct choice is "spectrum" because when a time-domain signal (or time waveform) is transformed using the Fast Fourier Transform (FFT), the resulting representation is a frequency-domain graph that displays the amplitude of different frequency components present in the original signal. This transformation allows for the analysis of the signal's frequency content, revealing how much of different frequencies are contained within that signal.

The spectrum provides critical insights in vibration analysis and other fields by highlighting peak frequencies, which can indicate resonances, operational characteristics, or possible faults in machinery. Understanding the frequency content of a signal is essential for diagnosing issues and making informed decisions regarding maintenance and repairs.

The other options do not accurately describe the result of an FFT transformation. While "chart" and "graph" are general terms for visual representations of data, they do not specifically pertain to the frequency analysis associated with FFT. Similarly, "waveform" refers to the time-domain representation of the signal before the transformation, not the frequency-domain result that is generated by applying the FFT.